The invention relates generally to semiconductor device and integrated circuit fabrication and, in particular, to device structures and fabrication methods for a BiCMOS integrated circuit.
Traditional complementary metal-oxide-semiconductor (CMOS) structures for a field-effect transistor include a source, a drain, a channel situated between the source and drain, and a gate electrode configured to respond to a gate voltage by selectively connecting the source and drain to each other through the channel. Field-effect transistor structures can be broadly categorized based upon the orientation of the channel relative to a surface of a semiconductor substrate with which they are formed. In a vertical field-effect transistor, the direction of the gated current flow in the channel between the source and drain is generally perpendicular (i.e., vertical) to the substrate surface.
A bipolar junction transistor is a three-terminal electronic device that includes an emitter, an intrinsic base, and a collector in its construction. The intrinsic base is arranged between the emitter and collector in the structure of the device. In an NPN bipolar junction transistor, the emitter and collector may be composed of n-type semiconductor material, and the intrinsic base may be composed of p-type semiconductor material. In a PNP bipolar junction transistor, the emitter and collector may be composed of p-type semiconductor material, and the intrinsic base may be composed of n-type semiconductor material. In operation, the base-emitter junction is forward biased, the base-collector junction is reverse biased, and the collector-emitter current may be controlled by the base-emitter voltage.
A heterojunction bipolar transistor is a variant of a bipolar junction transistor in which at least two of the collector, emitter, and intrinsic base in the device structure are constituted by semiconductor materials with different energy bandgaps, which creates heterojunctions. For example, the collector and/or emitter of a heterojunction bipolar transistor may be constituted by silicon, and the base of a heterojunction bipolar transistor may be constituted by a silicon germanium (SiGe) alloy, which is characterized by a narrower band gap than silicon.
Bipolar junction transistors may be combined with field-effect transistors in a bipolar complementary metal-oxide-semiconductor (BiCMOS) integrated circuit. Such a chip integration can leverage the favorable characteristics of both transistor types.
Improved device structures and fabrication methods for a BiCMOS integrated circuit are needed.